Electrostatic microactuator having a movable piece between a pair of statical members

ABSTRACT

An object of the present invention is to provide an electrostatic actuator that can be easily assembled and suitably mass-produced and that can implement requirements of both the steady, smooth and stable operation and the enhanced reliability. The present invention is also directed to a method of activating such an electrostatic actuator, and a camera module used with the same.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2001-283533, filed on Sep. 18,2001; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an electrostatic microactuator, amethod of activating the same, and a camera module used with the same,and more particularly, it relates to an electrostatic microactuatorelectrostatically powered, assembled easily, and having improvedsmoothness, stability, and reliability in actuation, a method ofactivating such an electrostatic microactuator, and a camera module usedwith the same.

For recent years, a miniaturized linear actuator of more downsizeddesign, precise operation and reduced cost has been increasingly neededfor focal adjustment of a super-compact camera, for example. An exampleof a solution to such needs is an electrostatic actuator disclosed inJapanese Patent Laid-Open Publication H08-140367.

FIG. 27 is a schematic diagram showing a structure of the prior artelectrostatic actuator.

As shown in FIG. 27, an electrostatic actuator 101 is comprised of amovable piece 102 and a couple of statical members 103 a and 103 boverlaid on the opposite sides of the movable piece. The staticalmembers 103 a and 103 b have their respective two groups of branch padsconnected to electrodes, and there are four groups of branch padsconnected to electrodes A to D for the couple of the upper and lowerstatical members.

Branch pads in the statical members 103 a and 103 b, which are connectedto corresponding ones of the electrodes A to D, are arranged at the samepitch with branch pads 104 of the movable piece 102, and all the branchpads are the same in width in both the statical and movable pieces. Inthe static pieces 103 a and 103, however, the branch pads separatelycorrelated with two of the electrodes (e.g., electrodes A and C) arealternately placed in an interlacing deployment. In addition to that,the branch pads of the upper and lower static pieces 103 a and 103 b arecorrelated with one another in a ½ out-of-phase pattern where the upperbranch pads are deviated by a half of their respective width from theircounterparts or the lower branch pads.

Applying high voltage to the electrode A, an electrostatic force(Coulomb force) developed between the electrode A and the branch pads104 correlated with an electrode E causes the movable piece to beattracted by the upper statical member 103 a (toward a position wherethe branch pads correlated with the electrodes A and E are aligned inphase). Then, switching the electrode supplied with the high voltage tothe electrode B, the movable piece 102 is attracted by the lowerstatical member 103 b (toward a position where the branch padscorrelated with the electrodes B and E are aligned in phase). In thisways the succeeding switching of the electrodes as in a manner of A toB, B to C, C to D, and so forth enables the movable piece 102 tomicroscopically vertically vibrate and macroscopically laterally move(e.g., move to the right in FIG. 27 with one degree of freedom).

Supplying the high voltage to the electrodes in the reversed order as inA to D, D to C, C to B, and so forth enables the movable piece to moveto the left in FIG. 27.

To implement such a way of the motion, the vertically juxtaposedstatical members 103 a and 103 b must be under accurate control of thephases or the branched-electrode pattern, and the movable piece 102 mustalso have an accurately fabricated electrode pattern on both theopposite sides. This requires time consuming and complicated assemblingtask and accordingly leads to a cost increase, which are some ofproblems that must be overcome for a mass-production of such a highprecision actuator.

Further, since the movable pieces in this electrostatic actuatorvibrates with a relatively large amplitude between the juxtaposedstatical members 103 a and 103 b to laterally move pitch by pitch, itsmicroscopic movement is not satisfactorily smooth, and it is desirableto improve both the physical and operational mechanisms of the actuator.

SUMMARY OF THE INVENTION

The present invention is made to address the aforementioneddisadvantages in the prior art. Accordingly, it is an object of thepresent invention to provide an electrostatic actuator that can beeasily assembled and suitably mass-produced and that can implementrequirements of both the steady, smooth and stable operation and theenhanced reliability, a method of activating the same, and a cameramodule used with the same.

According to an embodiment of the present invention, there is providedan electrostatic actuator comprising: a first statical member having anelectrode array being comprised of at least three groups of activatingelectrodes periodically deployed in a first direction; a second staticalmember faced to the first statical member and having an electrodeextending in the first direction; a movable piece provided between thefirst and second statical members, and a switching circuit applying afirst voltage to cause a potential difference between at least one ofthe groups of the activating electrodes and the movable piece and alsoapplying a second voltage to cause a potential difference between theelectrode of the second statical member and the movable piece, the firstvoltage being applied to sequentially switch a destination of appliedvoltage from at least one of the groups of the activating electrodes toanother in the first direction, the second voltage being intermittentlyapplied while the first voltage is applied.

According to an anther embodiment of the present invention, there isprovided an electrostatic actuator comprising: a first statical memberhaving an electrode array being comprised of at least three groups ofactivating electrodes periodically deployed in a first direction; asecond statical member faced to the first statical member and having afirst electrode extending in the first direction and a second electrodeextending in the first direction in almost parallel with the firstelectrode; a first movable piece provided between the first and secondstatical members, a second movable piece provided between the first andsecond statical members, and a switching circuit applying a firstvoltage to cause a potential difference between at least one of thegroups of the activating electrodes and the first movable piece and alsoapplying a second voltage to cause a potential difference between thefirst electrode and the first movable piece, the first voltage beingapplied to sequentially switch a destination of applied voltage from atleast one of the groups of the activating electrodes to another in thefirst direction, the second voltage being intermittently applied whilethe first voltage is applied, and the switching circuit applying a thirdvoltage to cause a potential difference between at least one of thegroups of the activating electrodes and the second movable piece andalso applying a fourth voltage to cause a potential difference betweenthe second electrode and the second movable piece, the third voltagebeing applied to sequentially switch a destination of applied voltagefrom at least one of the groups of the activating electrodes to anotherin the first direction, the fourth voltage being intermittently appliedwhile the third voltage is applied.

According to an anther embodiment of the invention, there is provided acamera module comprising: a imaging device; a electrostatic actuator,the electrostatic actuator including: a first statical member having anelectrode array being comprised of at least three groups of activatingelectrodes periodically deployed in a first direction; a second staticalmember faced to the first statical member and having an electrodeextending in the first direction; a movable piece provided between thefirst and second statical members, and a switching circuit applying afirst voltage to cause a potential difference between at least one ofthe groups of the activating electrodes and the movable piece and alsoapplying a second voltage to cause a potential difference between theelectrode of the second statical member and the movable piece, the firstvoltage being applied to sequentially switch a destination of appliedvoltage from at least one of the groups of the activating electrodes toanother in the first direction, the second voltage being intermittentlyapplied while the first voltage is applied; and a lens mounted on themovable piece of the electrostatic actuator and inputting a opticalinformation to the imaging device.

According to an anther embodiment of the invention, there is provided acamera module comprising: a imaging device; a electrostatic actuatorincluding: a first statical member having an electrode array beingcomprised of at least three groups of activating electrodes periodicallydeployed in a first direction; a second statical member faced to thefirst statical member and having a first electrode extending in thefirst direction and a second electrode extending in the first directionin almost parallel with the first electrode; a first movable pieceprovided between the first and second statical members, a second movablepiece provided between the first and second statical members, and aswitching circuit applying a first voltage to cause a potentialdifference between at least one of the groups of the activatingelectrodes and the first movable piece and also applying a secondvoltage to cause a potential difference between the first electrode andthe first movable piece, the first voltage being applied to sequentiallyswitch a destination of applied voltage from at least one of the groupsof the activating electrodes to another in the first direction, thesecond voltage being intermittently applied while the first voltage isapplied, and the switching circuit applying a third voltage to cause apotential difference between at least one of the groups of theactivating electrodes and the second movable piece and also applying afourth voltage to cause a potential difference between the secondelectrode and the second movable piece, the third voltage being appliedto sequentially switch a destination of applied voltage from at leastone of the groups of the activating electrodes to another in the firstdirection, the fourth voltage being intermittently applied while thethird voltage is applied; a lens mounted on the first movable piece ofthe electrostatic actuator and inputting a optical information to theimaging device; and a lens mounted on the second movable piece of theelectrostatic actuator and inputting a optical information to theimaging device.

Appropriately configured according to the present invention, a movablepiece, while being attracted and almost fitted onto a surface that hasbranch pads connected to a first electrode, is permitted to move,thereby restraining vertical vibration under strong attractive force, soas to attain the desired stable, smooth, and reliable operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given herebelow and from the accompanying drawings of theembodiments of the invention. However, the drawings are not intended toimply limitation of the invention to a specific embodiment, but are forexplanation and understanding only.

In the drawings:

FIG. 1 is a schematic diagram showing an embodiment of an electrostaticactuator according to the present invention;

FIG. 2 is a plan view showing a deployment of branch pads connected toelectrodes A to D in a first statical member 2 a;

FIG. 3 is a timing chart illustrating waveforms of voltage applied tothe electrodes by a switching circuit 40 in the electrostatic actuatorof the embodiment of the present invention;

FIG. 4 depicts a varied timing chart illustrating waveforms of voltageapplied to the electrodes by the switching circuit 40 in theelectrostatic actuator of the embodiment of the present invention;

FIG. 5 is a graph showing measurements resulted from motions of theelectrostatic actuator which is activated at the timing shown in FIG. 3;

FIG. 6 is a sectional view showing a mechanical structure of theelectrostatic actuator of the embodiment of the present invention;

FIGS. 7A and 7B are diagrams showing an exemplary electrostatic actuatorwhere vertically juxtaposed statical members 2 a and 2 b has a stopper10 and a movable piece 3 makes a frictional movement relative to thestopper 10;

FIGS. 8A and 8B are diagrams showing an exemplary electrostatic actuatorwhere the stopper 10 is placed in the movable pieces 3, respectively, toblock contact;

FIG. 9 is a graph showing a relation between the attracting forceeffected upon the movable piece 3 and a distance of the electrodes withapplied voltage of 150 V;

FIG. 10 is a graph showing a relation of the applied voltage with theattracting force developed between the branch pads and the movablepiece, and between the lower electrode and the movable piece with astopper height S of 3.5 microns;

FIG. 11 is a graph showing a relation of the applied voltage with theattracting force developed between the branch pads electrode and themovable piece, and between the lower electrode and the movable piecewith the stopper height S of 4.0 microns;

FIG. 12 is a graph showing a relation of the applied voltage with theattracting force developed between the branch pads and the movablepiece, and between the lower electrode and the movable piece;

FIG. 13 is a diagram showing a planar pattern of the lower electrode E;

FIGS. 14A and 14B are diagrams showing a concept of an operation of afourth exemplary electrostatic actuator of the embodiment of the presentinvention;

FIG. 15 is a timing chart illustrating voltage signals activating theactuator;

FIG. 16 is a timing chart illustrating a pattern of voltage applicationwhere the voltage between the branch pads and the movable piece isinverted to dissipate residual polarization from protective film 4;

FIG. 17 is a timing chart illustrating a pattern of voltage applicationwhere the voltage between the branch pads and the movable piece isinverted in a manner according to embodiment of the present invention todissipate the residual polarization from the protective film 4;

FIG. 18 is a diagram showing an application of a fifth exemplaryelectrostatic actuator of the present invention used for a lensmechanism in a camera module;

FIGS. 19A through 19C are diagrams showing an application of the fifthexemplary electrostatic actuator of the present invention used for alens mechanism in a camera module, where FIG. 19A is a cross-sectionalview of the actuator taken along its longitudinal axis, FIG. 19B is itsX—X cross-sectional view, and FIG. 19C is its Y—Y cross-sectional view;

FIG. 20 is a diagram showing a part of a compact camera moduleincorporated with the electrostatic actuator of the embodiment of thepresent invention;

FIGS. 21 through 23 are timing charts illustrating other examples of thetiming possibly used for the embodiment of the present invention;

FIG. 24 is a schematic diagram showing a basic structure of theelectrostatic actuator according to the embodiment of the presentinvention;

FIG. 25 is a timing chart illustrating waveforms of the voltage appliedto electrodes by a switching circuit 140;

FIG. 26 is a graph showing measurements resulted from monitoring themotion of a movable piece in the electrostatic actuator shown in FIG.24; and

FIG. 27 is a diagram showing a structure of the prior art electrostaticactuator.

DETAILED DESCRIPTION

In advance of detailing embodiments of the present invention, the basicsof an electrostatic actuator will be explained first.

FIG. 24 is a schematic diagram showing a structure of a basicelectrostatic actuator useful to understand the present invention.

The electrostatic actuator is comprised of first and second staticalmembers 2 a and 2 b opposed to each other and a movable piece positionedbetween them and capable of sliding along a direction as denoted by anarrow SL.

The first statical member 2 a has four groups of branch padsrespectively connected to electrodes A to D while the second staticalmember 2 b has a uniformly distributed electrode E in its surface. Aswill be recognized from the depiction in the figure, the four groups ofbranch pads correlated to the electrodes A to D are arranged in a stripedeployment where one from each group is succeedingly positioned oneafter another in the fixed order of A to D, for example, along adirection of advancement of the movable piece 3. A predetermined levelof voltage is applied from a voltage supply 142 through a switchingcircuit 140 to all of each group simultaneously.

FIG. 25 is a timing chart illustrating waveforms of the voltage appliedvia the switching circuit 140 to the branch pads correlated to each ofthe electrodes.

Initially, an electrode F connected the movable piece 3 keeps itspotential at a low level L, as shown in FIG. 25(f), while the electrodesA and B are supplied with voltage as shown in FIGS. 25(a) and 25(b). Anelectrostatic force between the electrodes A and B and the movable piece3 causes the movable piece 3 to be attracted to the branch pads A and Brespectively connected to those electrodes and almost fitted on thefirst statical member 2 a. Then, as shown in FIG. 25(e), the switchingcircuit 120 switches its connection from the electrodes A and B to anelectrode E so as to apply voltage to the electrode E, and the movablepiece 3 leaves the branch pads A and B, or it is attracted by andtowards the second statical member 2 b.

After that, the switching circuit 140 switches its connection from theelectrode E to the electrodes B and C so as to apply voltage to thebranch pads B and C connected to those electrodes as shown in FIGS.25(b) an 25(c), and an electrostatic force between the branch pads B andC and the movable piece 3 causes the movable piece 3 to be attracted bythe first statical member 2 a. Then, the switching circuit 140 switchesits connection from the branch pads B and C to the electrode E so as toapply voltage to the electrode E as shown in FIG. 25(c), andresultantly, the movable piece 3 leaves the branch pads B and C, or itis attracted toward the second statical member 2 b.

Next, the switching circuit 140 switches its connection from theelectrode E to the electrodes C and D so as to apply voltage to theelectrodes C and D as shown in FIGS. 25(c) and 25(d), and anelectrostatic force between the electrodes C and D and the movable piece3 results in the movable piece 3 being attracted by the first staticalmember 2 a. Subsequently, the switching circuit 140 switches theconnection from the electrodes C and D to the electrode E so as to applyvoltage to the electrode E as shown in FIG. 25(e), the movable piece 3leaves the electrode E, or it is attracted toward the second staticalmember 2 b.

Furthermore, the switching circuit 140 switches the connection from theelectrode E to the electrodes D and A to apply voltage to the electrodesD and A as shown in FIGS. 25(d) and 25(a), the resultant electrostaticforce between the electrodes D and A and the movable piece 3 causes themovable piece 3 to be attracted by the first statical member 2 a.Subsequently, the switching circuit 140 switches the connection from theelectrodes D and A to the electrode E to apply voltage to the electrodeE as shown in FIG. 25(e), and the movable piece 3 leaves the branch padsD and A, or it is attracted toward the second statical member 2 b.

Repeating the aforementioned operation sequence enables the movablepiece 3 to microscopically vertically vibrate and macroscopicallylaterally move along an extension of the branch pads in the firststatical member 2 a.

In the electrostatic actuator in FIG. 24, as has been described, thestatical member 2 b is provided simply with the uniformly distributedelectrode E, which permits a linear motion. Thus, unlike the prior artelectrostatic actuator having the electrodes in the stripe deployment inthe vertically juxtaposed statical members 103 a and 103 b as shown inFIG. 27, a precise phase control is needless over the branched-electrodepattern. A positional relation between the couple of the staticalmembers 2 a and 2 b does not have to be adjusted so precisely as in theprior art embodiment, and this leads to a time and labor saving forassembly and consequently brings about a cost reduction, so that amass-productivity can be greatly enhanced.

Additionally, in the electrostatic actuator in FIG. 24, since voltage isapplied alternately to the first and second statical members 2 a and 2b, the movable piece 3 laterally moves while vertically vibrating.

FIG. 26 is a graph showing measurements resulted from monitoring themotion of the movable piece in the electrostatic actuator in FIG. 24.The horizontal axis represents an elapse of time, and the vertical axison the left represents voltage applied to the electrodes while thevertical axis on the right represents a displacement of the movablepiece 3. Rectangles plotted in a lower half of the graph show a waveformof voltage applied to the electrodes while those in an upper half of thegraph show a vertical displacement of the movable piece 3, and lineextending from the lower left to the upper right in the graph alsorepresents a displacement of the movable piece 3 in directions of itsadvancement (lateral directions).

In FIG. 26, the lateral displacement is plotted at a reduction scale of{fraction (1/10)} to the vertical displacement. In other words, alateral displacement is actually ten times as long as a verticaldisplacement indexed at the same graduation line along the verticalaxis.

As can be seen in FIG. 26, each time a destination of the appliedvoltage is switched from one electrode or electrode pair to another, themovable piece 3 moves up or down while proceeding (in a lateraldirection) a little bit. An amplitude of such vertical movement dependsupon distance between the movable piece 3 and the juxtaposed staticalmembers 2 a and 2 b, and it is much greater than a lateral displacementequivalent to a single pitch of the movable piece. The movable piece 3,while vibrating with a relatively great amplitude, laterally advancesbit by bit. Moreover, the amplitude of the vertical vibration, althoughdefined as the distance between the movable piece 3 and the two of thestatical members 2 a and 2 b, is likely to vary due to manufacturingerror.

Since the Coulomb force developed between the movable piece 3 and thestatical members 2 a and 2 b is inversely proportional to the distancebetween those members that are raised to the second power, a continualvariation in the distance between those components may always affect theattractive force between them, and the motive force for the attractionand displacement of the movable piece becomes unstable.

The inventors' attempt was to improve such drawbacks in the prior art,and they have successfully enhanced operational smoothness and stabilityto complete a technically refined electrostatic actuator.

Embodiments of the present invention will be described in more detail.

(First Embodiment)

FIG. 1 is a schematic diagram showing a structure of an exemplaryembodiment of an electrostatic actuator according to the presentinvention. Like reference numerals as used in FIGS. 24 and 25 denotecorresponding components in FIG. 1.

The exemplary electrostatic actuator is comprised of first and secondstatical members 2 a and 2 b opposed to each other and a movable piece 3positioned between them and slidable in a direction designated by anarrow 24.

The first and second statical members 2 a and 2 b may be plate-like inshape, or alternatively be semi-cylindrical. When the first and secondstatical members 2 a and 2 b have a plate-like shape, the movable piece3 may accordingly be shaped like a solid or hollow block which has itsopposite almost flat surfaces faced with the statical members, or whenthe first and second statical members 2 a and 2 b have asemi-cylindrical shape, the movable piece 3 may correspondingly beshaped like a solid or hollow cylinder.

FIG. 2 is a plan view showing a deployment of groups of branch pads inthe first statical member 2 a respectively connected to electrodes A toD.

As can be seen in FIG. 2, the first statical member 2 a has four groupsof the branch pads A to D correlated to the electrodes designated by thecorresponding alphabetic symbols while the second statical member 2 bhas a uniformly distributed electrode E over a surface. The four groupsof the branch pads A to D are arranged as depicted in FIG. 2 to serve as“activating electrodes” in a stripe deployment where one branch pad fromeach group is succeedingly positioned one after another in the fixedorder along a direction of advancement of the movable piece 3. Apredetermined level of voltage is applied from a voltage supply 42through a switching circuit 40 to all branch pads of each groupsimultaneously.

As shown in FIG. 2, an application of the supply voltage, to the branchpads is switched from one to another in the fixed order, and since thebranch pads are used to advance the movable piece in a specifieddirection, those branch pads A to D will be referred to as “activatingelectrodes” hereinafter. The uniformly distributed electrode E providedin the second statical member 2 b will also be referred to as “lowerelectrode” hereinafter. The lower electrode may be discrete if discreteelectrode elements are uniformly distributed and are of the same kind.

The reference numeral 10 in FIG. 2 denotes a stopper defining a fixedclearance between the movable piece 3 and the activating electrodes.This will be detailed later.

FIG. 3 is a timing chart illustrating waveforms of voltage applied tothe electrodes through the switching circuit 40 in the electrostaticactuator according to the present invention.

The timing can be analyzed and expressed briefly as “upon an applicationof voltage to the lower electrode, any of the activating electrodes aresimultaneously supplied with voltage”. Applying voltage at the timing asdepicted in the chart permits the movable piece 3 to laterally advancewhile being attracted and almost fitted onto the first statical member 2a.

Regarding the timing exemplified in FIG. 3, initially, an electrode Fconnected to the movable piece 3 keeps its potential at a low level asshown in FIG. 3(f) while the activating electrodes A and B are suppliedwith voltage as shown in FIGS. 3(a) and 3(b). Thus, an electrostaticforce between the activating electrodes A and B and the movable piece 3causes the movable piece 3 to be attracted by the activating electrodesA and B and almost fitted onto the first statical member 2 a.

Then, simultaneous with the timing of turning the activating electrode Ato the low level, the lower electrode E has its potential turned to thehigh level. During this stage, the voltage applied to the activatingelectrode B keeps at the high level. Thus, the movable piece 3, which iscontinuously attracted and almost fitted onto the first statical member2 a having the “activating electrode” activated, is also attracted bythe lower electrode E and forced to insufficiently free from the firststatical member 2 a. The movable piece 3 creepingly floats laterallytoward the activating electrode B. The activating electrode B in thisstage serves as “assisting electrode” that disturbs the lower electrodeE from capturing the movable piece 3 with its attractive force.

After that, while the activating electrode B keeps its potential at thehigh level, the activating electrode C is supplied with voltage to thehigh level, and the lower electrode E has its potential turned to thelow level. Resultantly, the movable piece is attracted by the firststatical member 2 a while creeping laterally toward the activatingelectrode C.

Subsequently, while the activating electrode C keeps its potential atthe high level, the activating electrode B has its potential turned tothe low level, and the lower electrode E is supplied with voltage to thehigh level. As a result, the movable piece 3, while continually beingattracted and almost fitted onto the activating electrodes, is alsoattracted by the lower electrode E and forced to insufficiently freefrom the first statical member 2 a, and it creepingly floats toward theactivating electrode C. Thus, the activating electrode C similarlyserves as the “assisting electrode”.

After the activating electrodes C, D, and A are sequentially suppliedwith voltage in the similar procedure, one cycle is completed. Thesimilar cycle is repeated for the subsequent series of the activatingelectrodes A to D, and consequently, the movable piece 3, whilecontinuously kept almost fitted onto the first statical member 2 a bythe attractive force, can move laterally.

In this way, simultaneous with the application of voltage to the lowerelectrode E, any of the activating electrodes A to D is supplied withvoltage to serve as the “assisting electrode”, and this effectivelyrestrains the vertical vibration of the movable piece 3 and permits themovable piece almost fitted onto the activating electrode to advancesmoothly.

FIG. 4 is a timing chart of a variation of the electrostatic actuatoraccording to the embodiment of the present invention, showing waveformsof the voltage applied to the electrodes. In this variation, theadjacent ones of the electrodes such as the electrodes A and B aresupplied with voltage in a ON/OFF pattern as depicted in the initialsegment of the chart where one of the electrodes is turned on and thenimmediately turned off, and this succeeding procedure is repeated fourtimes. The electrode E has its voltage level switched in accord with thesucceeding ON/OFF. This ON/OFF pattern is followed by the next adjacentones of the electrodes such as the electrodes B and C, and after thesucceeding procedure is repeated four times, the same ON/OFF pattern isfollowed by the further next adjacent ones of the electrodes such as theelectrodes C and D. This is further followed by the succeeding electrodecouples D and A, and so on.

Repeatedly turning on and off the adjacent ones of the electrodes in theaforementioned manner can ensure the advancement of the movable piece.In the single ON/OFF pattern, one of the electrodes is turned on andthen immediately turned off four times, but the number of times of thesucceeding procedure should not be limited; that is, the frequency ofthe turning on and off can be determined as desired in each application,and the pattern may be repeated more or less frequently than four times.

FIG. 5 is a graph showing measurements resulted from monitoring themotion of the movable piece when the electrostatic actuator is activatedat the timing as depicted in FIG. 4. The horizontal axis represents thetime, and the vertical axis on the left shows voltage applied to theelectrodes while the vertical axis on the right shows a displacement ofthe movable piece 3. Rectangles plotted in a lower half of the graphshow a waveform P of voltage applied to the electrodes while those in anupper half of the graph show a vertical displacement V of the movablepiece 3, and line extending from the lower left to the upper right inthe graph also represents a lateral displacement H of the movable piece3 in directions of its advancement.

In FIG. 5, the lateral displacement is plotted at a reduction scale of{fraction (1/10)} to the vertical displacement. In other words, alateral displacement is actually ten times as long as a verticaldisplacement indexed at the same graduation line along the verticalaxis.

As will be recognized in FIG. 5, after attracted toward the activatingelectrodes at the initial timing, the movable piece 3 moves laterallywhile continuously keeping almost fitted onto the activating electrodes.Applying voltage alternately to the activating electrodes and the lowerelectrode, the movable piece 3 moves laterally while verticallyvibrating.

In contrast, in the embodiment of the present invention, voltage issimultaneously applied to the lower electrode and the activatingelectrodes. In this manner, as depicted in FIG. 5, the movable piece 3is permitted to move smoothly in lateral directions while continuouslyattracted and almost fitted onto the activating electrodes. As aconsequence, undesired vertical vibration can be restrained, and anadditional effect similar to that which is attained in the condition ofreduced clearances between the movable piece 3 and the statical member 2a can be obtained; that is, the motive force for the attraction anddisplacement of the movable piece is advantageously enhanced. Thus,since the clearances between the activating electrodes A to D and themovable piece 3 can keep minimized, the sufficient and stable attractingforce or Coulomb force effects upon the movable piece 3.

In the embodiment shown in FIG. 5, one of the adjacent electrodessucceedingly and repeatedly turns on and then off four times. It shouldbe noted that the first turning on and off causes the movable piece 3 tolaterally move, and the remaining ON/OFF actions repeated three timescause almost no lateral displacement of the movable piece 3. This isbecause the initial turning on and off on the adjacent electrodes (e.g.,the electrodes A and B) forces the movable piece to fall in a standoffzone of the activated electrodes. On the contrary, when some disturbancesuch as frictional force on the movable piece 3 acts to keep the movablepiece 3 off the standoff zone even after the initial turning on and off,it is likely for the second or even succeeding turning on and off cancause the movable piece to reach the standoff zone.

(Second Embodiment)

A second embodiment of the electrostatic actuator will now be describedin the context of an improved feature of adjusting a balance of voltageapplied to the activating electrodes and the lower electrode.

The inventors reviewed the first embodiment of the electrostaticactuator and obtained some quantitative observations on the clearancesbetween the statical member 2 a and the movable piece 3 and the voltageapplied to the electrodes.

A mechanical structure will be outlined for comprehensive recognitionsof this embodiment.

FIG. 6 is a sectional view showing the mechanical structure of theelectrostatic actuator of the embodiment of the present invention. Theactivating electrodes A to D and the lower electrode E have theirrespective operation surfaces covered with protective film 4,respectively. The protective film 4 is of insulating material such asinorganic composites including silicon oxide and silicon nitride, andorganic composites including polyimide.

A stopper 10 is provided to avoid direct contact of the protective film4 with the movable piece 3. For instance, as shown in FIGS. 7A and 7B,the upper and lower pieces 2 a and 2 b respectively have the stopper 10in their surface to permit the movable piece 3 to acts frictionally onthe stopper 3, and thus, the movable piece 3 can keep off the protectivefilm 4. Alternatively, as illustrated in FIGS. 8A and 8B, a contactbreaking element such as the stopper 10 may be placed on the movablepiece 3.

Turning to FIG. 6, again, to proceed with the discussion, there aregiven sample values of a height of the stopper and a width of theclearance. A height S of the stopper may be approximately 4.5 microns.An amplitude value V in which the movable piece 3 can traverse may be4.0 microns.

Also, FIG. 6 depicts the movable piece 3 attracted and almost fitted onthe activating electrodes A to D, and this posture minimizes aninter-electrode distance, namely, a distance between the movable piece 3and the activating electrodes A to D. The inter-electrode distance(minimum) G1 may be approximately 4.0 microns. A height of theactivating electrodes A to D may be approximately 0.5 microns. Undersuch circumstances, another inter-electrode distance, namely, a distancebetween the movable piece 3 and the lower electrode E is maximized. Thisinter-electrode distance (maximum) G2 may be 8.0 microns.

Taking a longitudinal dimension in consideration, when, for onereference, projections 3 a of the movable piece 3 are stationed havingtheir respective center positioned in the activating electrode A, alength of the projections 3 a and pitches of the sequence of theactivating electrodes A to D can be determined by fixing an amount ofoverlap between each projection 3 a and the activating electrode B toapproximately 6.0 microns.

FIG. 9 is a graph showing a relation between an attractive force on themovable piece 3 and the inter-electrode distances. In this graph, thehorizontal axis represents the inter-electrode distance between themovable piece 3 and the activating electrodes A to D while the verticalaxis represents the attractive force on the movable piece 3. Also, inFIG. 9, the inter-electrode distance (maximum) G2 is fixed at 8 micronswhile the height S of the stopper is varied, and the attractive force ismonitored in relation with the varied distance between the movable piece3 and the activating electrodes A to D.

In this case, it is assumed that the movable piece 3 keeps attracted andalmost fitted onto the activating electrodes A to D, as depicted in FIG.6. Thus, the distance between the movable piece 3 and the activatingelectrodes A to D represented on the horizontal axis is identified withthe inter-electrode distance (minimum) G1. The value is equal to anamount of the height S of the stopper minus the thickness of each of theelectrodes A to D.

The force developed between the movable piece 3 and the activatingelectrodes A to D, and between the movable piece 3 and the lowerelectrode E can be expressed by the following formula:$F = {ɛ\frac{{SV}^{\quad 2}}{2d^{2}}}$

where ε is a dielectric constant (8.85×10⁻¹² F/m in vacuum condition), Sis an area of each activating electrode, V is an applied voltage, and dis an inter-electrode distance (clearance), respectively.

When a distance between the movable piece 3 and the electrodes A to E isvaried in the course of the operation, there arises ripple in outputacross the electrodes. A value of the inter-electrode distance (maximum)G2 affects a minimum value of the output ripple, and if the G2 israised, the output ripple (minimum) is decreased. Thus, FIG. 9 depictsthe relation of the movable piece 3 with the attractive force under theassumption that the inter-electrode distance (maximum) G2 is fixed whilethe height S of the stopper is varied.

In FIG. 9, when the inter-electrode distance (minimum) G1 is smallerthan 3.5 microns, the attractive force from the activating electrodes Ato D upon the movable piece 3 is higher than that from the lowerelectrode E upon the same, and if the inter-electrode distance G1 risesbeyond 3.5 microns, the former relation is reversed, and the attractiveforce from the lower electrode E is greater.

When the movable piece 3 is activated while continually kept attractedby the activating electrodes A to D, the inter-electrode distance(minimum) G1 must be lower than 3.4 microns. This means that the upperlimit of the height S of the stopper is approximately 4.0 micron.

Allowing for the attractive force upon the movable piece 3, it isdesirable the height S of the stopper is reduced. Actually, when theheight S of the stopper is reduced, however, it should possibly occurthat a contact of the movable piece 3 with the protective film 4 causesdeterioration and failure. A reduction of the inter-electrode distance(minimum) G1 causes an increase in an intensity of the electric field,and hence, discharge breakdown of the protective film is more likely tocause deterioration and failure. When the protective film 4 is damaged,it is more likely to undergo discharge breakdown, and hence, the damageand the discharge breakdown, when they are the cause of each other,heightens a potential risk of deterioration and failure. In view ofreliability of any practical use in various applications, this is amatter of importance.

Thus, it is critical that structure parameters and activation conditionsare appropriately determined as required in specifications for any useand application of the electrostatic actuator so that the electrostaticactuator can attain the maximum attractive force as possible whileensuring its operation reliability.

Referring to FIGS. 10 to 12, an example of the procedure will beexplained.

FIG. 10 is a graph showing relations of the applied voltage to theattractive force acting between the activating electrode and the movablepiece, and between the lower electrode and the movable piece, under thecondition of the height S of the stopper of 3.5 microns. In FIG. 10, thedimensional parameters as exemplified in FIG. 6 are used, and the heightS of the stopper is 3.5 microns. Thus, the inter-electrode distance(minimum) G1 is 3.0 microns.

With this height of the stopper and with voltage applied in a rage from100 V to 200 V, the attractive force from the activating electrodes A toD upon the movable piece 3 is always higher than that from the lowerelectrode E upon the movable piece 3. Naturally, the movable piece 3 isactivated while continuously attracted toward the activating electrodesA to D. When the reliability of the actuator and the precision ofmachining components tolerate this height S of the stopper, there arisesno problem due to the height. For example, once determined that themovable piece would not come in contact with the protection film 4 in arange of the varied height that is presumed based upon the precision ofmachining the stopper 10, it is possible to use the height in the range.

FIG. 11 is a graph illustrating relations of the applied voltage to theattractive force acting between the activating voltage and the movablepiece, and between the lower electrode and the movable piece. In a caseof the figure, with the height S of the stopper of 4.0 microns, theattractive forces match between the activating electrode and the movablepiece, and the lower electrode and the movable piece, and this leavesthe movable piece unstable.

Specifically, under the conditions, the movable piece 3 is not alwaysattracted toward the activating electrodes A to D, and sometimes it maybe attracted and almost fitted onto the lower electrode E to exhibitvertical vibration.

FIG. 12 is a graph showing the relations of the applied voltage to theattractive force between the activating electrodes and the movablepiece, and between the lower electrode and the movable piece under thecondition of the height S of the stopper of 4.5 microns. In the graph,it is apparent that with the height S of the stopper of 4.5 microns, therelations between the activating electrodes and the movable piece andbetween the lower electrode and the movable piece are switched andreversal.

It seems that if operating waveform with timing assist is given, themovable piece 3 vertically vibrates in accordance with given timing thatdepends upon an instructed voltage level. This would never promise anenhancement of stable and sufficient actuating performance.

On the other hand, in view of the precision of component machining andthe device reliability, sometimes it is desired that the height S of thestopper is predetermined higher than usual as in FIGS. 11 and 12.Accordingly, in the embodiment of the present invention, voltage appliedto the lower electrode E is regulated to have a stable operation.

This manner of stabilization will be discussed below with reference toFIG. 12.

It is now assumed that the voltage applied to the activating electrodesA to D is determined in advance to be 150 V. Under the condition, thelower electrode E may be adjusted to keep the attractive force from thelower electrode E smaller than that from the activating electrodes A toD.

Specifically, what to do fist is to get a value of the attractive forcebetween the activating electrodes and the movable piece when 150 V isapplied. It is approximately 12.5 mN in FIG. 12. Then, a voltage levelat the lower electrode required for developing the same level of forceas the attractive force must be known. In FIG. 12, it is recognized asbeing approximately 130 V. If the known voltage or voltage smaller isapplied to the lower electrode E, the attractive force from theactivating electrodes A to D is always higher, and the movable piece 3can be activated while continually keeping attracted and almost fittedonto the activating electrodes A to D.

In this embodiment, regulating the relation between the applied voltageto the activating electrodes A to D and that to the lower electrode E tokeep the desired state where the attractive force from the activatingelectrode is always higher, so that the stable attractive force actsupon the movable piece without vertical vibration of the same.

(Third Embodiment)

A third embodiment of the present invention will be described, which isan electrostatic actuator having a well-balanced electrode area of theactivating electrodes relative to the lower electrode.

In this embodiment, instead of adjusting voltage applied to each of theelectrodes as described regarding the second embodiment, areas of theupper and lower electrodes are regulated relative to each other to keepthe well-balanced state of the attractive force between thoseelectrodes.

For example, the activating electrodes A to D are deployed in repeatedstripes as shown in FIG. 2, and voltage is applied to a specificgroup(s) of the electrodes. In response to this, the lower electrode Edevelops attractive force.

As to the operation sequence illustrated in FIG. 3, only one group ofthe same branch electrodes A, B, C or D are supplied with voltagesimultaneous with an application of voltage to the lower electrode E.Thus, a face-to-face area of all the activating electrodes in the samegroup to the movable piece 3 should be adjusted to have a desiredvalance with the face-to-face area of the lower electrode E to themovable piece 3.

In an alternative simplified manner, the face-to-face area of theactivating electrodes of the same group to the movable piece 3 may belarger than the face-to-face area of the lower electrode E to themovable piece 3. A rate of one of the face-to-face area to the other maybe determined as required, depending upon the applied voltage and theheight S of the stopper.

The point is that the movable piece 3 should be activated while keepingcontinuously attracted and almost fitted onto the activating electrodesA to D.

FIG. 13 is a schematic diagram illustrating a plane pattern of the lowerelectrode E. The lower electrode E is shaped in three parallel stripes,respectively extending in an advancement direction of the movable piece3. A width W of each strip is varied to adjust the total area of thelower electrode E.

Thus, besides the parameters such as the height S of the stopper and thevoltage applied to the electrodes A to E, the stripe width W of thelower electrode E should be varied so that the activating electrodesalways exert greater attractive force. Consequently, the stableattractive force acts upon the movable pieces 3 to move it withoutvertical vibration.

(Fourth Embodiment)

Now, a fourth embodiment will be described, which is an electrostaticactuator capable of somewhat preventing an adverse effect by dielectricpolarization of the protection film 4.

FIGS. 14A and 14B illustrate a concept of the operation of the exemplaryelectrostatic actuator according to the embodiment of the presentinvention.

FIG. 15 is a timing chart of a voltage signal used to activate theactuator.

As can be seen in FIGS. 14A and 14B, there are three groups of theactivating electrodes A to C, and the lower electrode D is positionedopposite to them. The number of the groups of the activating electrodes,however, is not limited to three, but there may be four groups of them,or rather there may be five or more of them.

In this embodiment, for example, applying voltage of reversed polarityto the activating electrodes A to C restrains an influence of thedielectric polarization in the protective film 4.

In the timing charts of FIG. 15, FIGS. 15(a), 15(b), and 15(c) representvoltage signals applied to the branched electrodes A, B, and C,respectively. FIG. 15(d) represents a voltage signal applied to theelectrode D connected to the statical member while FIG. 15(e) shows avaried level of voltage applied to the electrode E connected to themovable piece.

Voltage applied to the electrode E in FIG. 15(e) is at ground potential.Voltage applied to the electrode D in FIG. 15(d) is switching betweenhigh H and low L, and the latter is at ground potential. Voltage signalsapplied to the electrodes A, B and C in FIGS. 15(a), 15(b) and 15(c) arealso switching between high H and low L where the former is at apositive potential while the latter is at a negative potential, andtheir average level is at ground potential.

Thus, the movable piece 3 is attracted to the electrodes A, B, and C athigh when the signals applied to them are turned to high H, and themovable piece 3 is repelled from the electrodes when the signals appliedto them are turned to low L. The electrodes A, B and C at the potentialof the intermediate level do not affect the movable piece 3.

As shown in FIG. 14A, for instance, when positive level voltage isapplied to the electrode B, electrostatic force (Coulomb force) affectsthe movable piece 3, and it is attracted toward the statical member 2 a.For convenience of explanation, FIGS. 14A and 14B depicts voltage beingapplied to only the group of the electrodes B, but as shown in thetiming charts in FIG. 15, voltage may be applied to the remainingelectrode A or C simultaneously.

When positive level voltage is applied to the electrode B as depicted inFIG. 14A, the protective film 4 at the surface of the activatingelectrode B cause dielectric polarization 5, and macroscopically, theelectrode B behaves to the movable piece 3 as if it were at positivepotential at its surface.

The dielectric polarization 5 in the protective film 4 sometimes resideseven after the application of positive voltage to the electrode B isinterrupted. This causes the movable piece 3 to stay continuouslyattracted close to the electrode B. Because of this, at the succeedingtiming, the movable piece 3 may sometimes lose propelling force thaturges it smoothly move to the adjacent electrode C.

This is resulted from an electrical deflection caused by the dielectricpolarization in the protective film 4. Although the dielectricpolarization 5 diminishes the residual potential, Coulomb force betweenelectrodes is inversely proportional to a distance raised to the secondpower, and therefore, in a situation where once the movable piece 3 isattracted toward the electrode B, and the inter-electrode distance comesclose to its minimum, the residual potential in the protective film 4,even if it is very low, gives considerable influence upon the movablepiece 3. Since such an adverse effect is reduced in this variation ofthe embodiment, the operation sequence allows for the desired motion ofthe movable piece 3.

In this embodiment, during a transition of the operation sequence from astate where the movable piece 3 is attracted by the electrode B to astate where the movable piece is to move toward the electrode C, apotential difference is provided between the electrodes B and E so thatthe electrode B has a lower potential than the electrode E connected themovable piece 3 (when the movable piece 3 is at zero level of potential,the electrode B exhibits a negative potential). This results in themovable piece 3 easily leaving the protective film 4, and a smoothoperation of the actuator can be attained.

Macroscopically, this can be understood well in the context of asituation of the electrode B connected to the statical member 2 a andthe electrode E connected to the movable piece 3 by the explanation asfollows: An electric field caused by the deflection of electric chargeresiding in the protective film 4 due to the dielectric polarization 5and an electric field caused due to the potential difference between theelectrodes B and E (potential of B is lower than that of E) are reversedin direction to each other and cancel each other. Microscopically, thismay also be taken as a phenomenon that the deflection of electric chargeresiding in the protective film 4 due o the dielectric polarization 5 isfaded to naught by the electric field caused by the potential added tothe electrode B (lower than the potential of the electrode E).

In the aforementioned embodiment, the potential at the movable piece 3may be electrically floated without being grounded. Alternatively, adummy electrode may be positioned proximal to the movable piece 3 andgrounded so that the electrostatic force effects well upon the movablepiece 3. In the embodiments shown in FIGS. 4(a) and 4(b), the protectivefilm 4 is placed on the statical member 2 a, and alternatively, it maybe provided in the movable piece 3, or rather, the protective film 4 maybe provided in both the statical member 2 a and the movable piece 3,respectively.

Reversing a polarity of the voltage applied to the activating electrodesis not the only manner of erasing the residual polarization in theprotective film 4. The similar result of reversing a polarity of theelectric field applied to the protective film 4 can be obtained byswitching the voltage related status between the activating electrodesand the movable piece 3 to dissipate the residual polarization.

FIGS. 16 and 17 are timing charts illustrating a pattern of appliedvoltage that effective to dissipate the residual polarization in thefilm 4 by switching the voltage related status between the activatingelectrodes and the movable piece. FIG. 16 represents a timing in asample embodiment while FIG. 17 represent a timing according to theembodiment of the present invention.

Referring to FIG. 16, at the initial time t1, voltage is first appliedto the electrodes A and B without application of voltage to the movablepiece 3. Then, at time t2, the electrodes A and B are turned off whileinstead, voltage is applied to the lower electrode. After that, at timet3, simultaneous with an application of voltage to the electrodes C andD, voltage is also applied to the movable piece 3. Since there is novoltage application to the electrodes A and B at time t3, electric fieldapplied to the protective film 4 on the electrodes A and B is reversedin contrast to the time t1 when voltage is applied to the electrodes Aand B.

The voltage relation between the electrodes A and B is switched to thereverse state from the timing t1 to the timing t3, and the electricfield applied to the protective film 4 is accordingly reversed. In thisway, the reversal of the electric filed enables the protective film 4 todissipate the residual polarization therein.

A timing pattern used to dissipate the residual polarity according toembodiment of the present invention is shown in a timing chart in FIG.17. In FIG. 17, at any time when the lower electrode is supplied withvoltage, voltage is applied to specific ones of the activatingelectrodes A to D to avoid vertical vibration of the movable piece 3.Appropriately applying voltage to the movable piece 3 also switches thevoltage relation between the activating electrodes A to D and themovable piece 3, and additionally, appropriately reversing the electricfield applied to the protective film 4 enables it to dissipate theresidual polarization therein.

In the aforementioned embodiments in FIGS. 14A through 15, althoughthree levels of voltage or positive, negative and ground levels arerequired, the embodiments shown in FIGS. 16 and 17 advantageouslyprovides a manner of dissipating the residual polarity in the protectivefilm 4 simply by two levels of voltage, or positive (or negative) andground levels.

(Fifth Embodiment)

A fifth embodiment of the electrostatic actuator having a plurality ofmovable pieces will now be described.

FIGS. 18 through 19C are schematic views showing an application of theexemplary electrostatic actuator to a lens mechanism of a camera module.

The lens mechanism includes a cubic hollow statical member 20 and twomovable pieces 30 and 40 reciprocally movable along a longitudinal axisof the statical member 20. Additionally, a camera module including thelens mechanism is incorporated with a CCD device 50, which is positionedat one end of the statical member 20 to detect an image. A lens isdepicted in alignment with the optical axis.

The statical member 20 has a frame 20, an electrode 22 patterned in aglass substrate through semiconductor processing technology andconnected to the statical member, projections 25 extending in adirection C parallel to the optical axis within the frame 21, and apower supply 26 for the electrode 22.

The statical electrode 22 has groups of activating electrodes 23 andgroups of lower electrodes 24.

The activating electrodes 23 include four groups of activatingelectrodes 23 a to 23 d, one electrode from each group is alternatelypositioned in series in the direction C. The activating electrodes 23 ato 23 d are deployed in stripes perpendicular to the direction C. Thenumber of the groups of the activating electrodes 23 is not limited tofour, but there may be three of the groups, or even five or more of thegroups of the activating electrodes.

On the other hand, there are two groups of the lower electrodes 24 a and24 b, and they are deployed in stripes in the direction C parallel tothe optical axis.

The power supply 26 includes a power supply circuit 26 a and a switchingcircuit 26 b. The power supply circuit 26 a selectively applies voltageto the two groups 24 of the lower electrodes 24 a and 24 b to hold oneof the movable pieces 31 and 32 as a standby and inhibit it from movingin the direction C.

The switching circuit 26 b applies voltage alternately to the activatingelectrodes 23 a to 23 d and the lower electrodes 23 a and 24 b otherthan the satndbys and succeedingly switches the destination of voltagesupply among the activating electrodes 23 a to 23 d in order.

Application of voltage to the electrodes is carried out at the timing asmentioned above in conjunction with the first to fourth embodiments ofthe present invention. Simultaneous with application of voltage to thelower electrodes 24 a and 24 b, specific ones of the activatingelectrodes 23 a to 23 d are necessarily supplied with voltage, and themovable pieces 31 and 32 are activated while keeping continuouslyattracted and almost fitted onto the activating electrodes 23.

The movable piece 31 includes a group of lenses 31L supported by a bodyof the movable piece, passive electrodes 31 affected by activation forcederived from the voltage at the electrode 22, and electrodes 31 b.

A plurality of the passive electrodes 31 a are deployed in stripes in adirection perpendicular to the optical axis or the direction C. Theelectrodes 31 b face the lower electrodes 24 a and are deployed instripes in the direction C parallel to the optical axis.

The movable piece 32 includes a group of lenses 32L supported by thebody of the movable piece, passive electrodes 32 a affected byactivation force derived from the voltage at the electrode 22, andelectrodes 32 b.

The passive electrodes 32 b are deployed in stripes perpendicular to theoptical axis or the direction C. The electrodes 32 b face the lowerelectrodes 24 b and are deployed in stripes in the direction C parallelto the optical axis.

The lens mechanism configured in this manner works in a fashion asdescribed below. For simplification, herein, a case where only themovable piece 31 is moved in a direction designated by an arrow α inFIG. 19A. FIG. 19A shows the movable piece 31 being in its initialposition. The sequence of activating the movable piece follows thetiming illustrated in FIG. 3.

When the switching circuit 26 b applies voltage V to the activatingelectrodes 23 a and 23 b, electrostatic force and attractive force aredeveloped between the activating electrodes and the passive electrode 31a of the movable piece 31 and between those electrodes and the passiveelectrode 32 a of the movable piece 32. The attractive force derivedfrom the activating electrodes 23 a and 23 b causes the first and secondmovable pieces 31 and 32 toward the activating electrodes 23 on thestatical member 20.

The switching circuit 26 b turns the voltage at the activating electrode23 a to low, and simultaneously apply voltage to the lower electrodes 24a and 24 b. The movable pieces 31 and 32, while keeping continuouslyattracted and almost fitted onto the activating electrodes, slightlymoves toward lateral directions due to downward attractive force in theplane of FIGS. 19A through 19C.

Continually supplying the lower electrode 24 b with voltage through thepower supply circuit 26 a enables the movable piece 32 to stay in itsinitial position. At this time, determining a level of the voltageapplied to the lower electrode 24 b to be higher than that foractivating the movable piece 32, it is ensured that the lower electrode24 b can securely hold the movable piece 32.

Then, the activating electrodes 23 b and 23 c are supplied with voltagethrough the switching circuit 26 b, and subsequently, the activatingelectrode 23 b is turned to low while the lower electrode 24 a issupplied with voltage.

Applying voltage to the electrodes following the timing pattern asillustrated in FIG. 3 permits the movable piece 31 to move in adirection a denoted in an arrow while keeping attracted and almostfitted onto the activating electrodes 23.

Since the power supply circuit 26 a keep supplying the lower electrode24 b with voltage, the movable piece 32 is securely held in its initialposition without moving.

In this way, the switching circuit 26 b applies voltage alternately tothe activating electrodes 23 a to 23 d of the electrode groups 23 andthe lower electrode 24 a of the electrode groups 24, and the order ofvoltage supply is appropriately determined among the electrodes andelectrode groups. Thus, the movable piece 31 can advance in thedirection C, and with the power supply circuit 26 a continuallysupplying the lower electrodes 24 b with voltage, the movable piece 32can be secured in a fixed position.

When only the movable piece 32 is to be moved, the switching circuit 26b applies voltage alternately to the activating electrodes 23 a to 23 dof the electrode groups 23 and the lower electrodes 23 b. In this way,the similar operation of the components can be attained, and with thepower supply circuit 26 a continually supplying the lower electrodes 24b with voltage, the movable piece 31 can be secured in its initialposition.

In any case, when the movable piece need to be fixed in position,voltage higher than that is required to move the movable piece should beapplied to the lower electrodes 24 a and 24 b. In this way, it isensured that the movable piece can securely be held.

In this embodiment, also, in order to avoid useless vertical vibrationof the movable piece 31 when the movable piece 31 is to be activated,the voltage applied to the activating electrodes 23 a to 23 d and thatapplied to the lower electrodes 24 a should be appropriately adjusted asin the aforementioned second embodiment. Alternatively, as described inconjunction with the third embodiment, a rate of areas of the activatingelectrodes 23 a to 23 d to the lower electrode 24 a may be appropriatelyvaried to get the desired balanced state.

The statical member 20 is provided with the projections 25 to reduce“idling” or “clattering” during the activation of the movable pieces 31and 32, and the projections are in line contact with the movable pieces31 and 32. Thus, the projections 25 serve as line contact supports forthe movable pieces 31 and 32.

According to the embodiment of the present invention, following theoperation sequence as aforementioned in conjunction with the first tofourth embodiments, the movable pieces 31 and 32 can be moved inparallel with the optical axis while keeping continuously attracted andalmost fitted onto the activating electrodes 23. As a result, undesiredvertical vibration of the movable pieces 31 and 32 can be avoided, andfrictional resistance caused by a contact of the movable pieces 31 and32 with the projections 25 can be considerably reduced. Thus, anactivation efficiency can be enhanced.

As has been described, in the lens mechanism incorporated with theelectrostatic actuator according to the embodiment of the presentinvention, the plurality of the movable pieces 31 and 32, which hold anarray of the groups of lenses, can be selectively moved, and thus, anactuator design having a zooming feature of lens driving mechanism canbe implemented.

Frictional resistance caused between the statical member 20 and themovable pieces 31 and 32 can be considerably reduced, and hence, anoperation efficiency can be enhanced.

Although, in the above mentioned embodiment, that which includes twomovable pieces has been described, the invention is not limited to this,but an actuator having three of the movable pieces can be similarlyconfigured and obtain the same effects.

In this way, providing the lower electrodes dedicated to theirrespective movable pieces, and appropriately applying voltage, selectedone of the movable pieces can be held in a fixed position while only theremaining can be moved in parallel with the optical axis.

This embodiment can be applied not only to the lens mechanism but alsoto a variety of mechanisms where a plurality of movable pieces are to bemoved independent from each other.

(Sixth Embodiment)

A sixth embodiment will now be described, which is a compact cameramodule which may have any of the first to fifth embodiments of theelectrostatic actuator built in.

The electrostatic actuator according to the present invention isexcellent in activation property, and therefore, it is suitable for usein focusing and/or zooming mechanism in a compact camera.

FIG. 20 is a schematic diagram illustrating a compact camera modulewhich have the electrostatic actuator of the embodiment of the presentinvention built in. In the compact camera in FIG. 20, there is animaging device such as CMOS, CCD, or the like on a substrate 321, andthe electrostatic actuator 322 such as any of the first to fifthembodiments of the present invention is positioned right in front of theimaging device.

The movable piece of the electrostatic actuator may be integrated withlens as has been described in conjunction with the fourth embodiment. AnIC of DSP (digital signal processor) is mounted on the substrate 321 tocontrol operation of the electrostatic actuator.

Such a camera module is used as a camera unit which is compatible withcellar phone, digital camera, portable PC, and the like.

With any of the first to fifth embodiments of the electrostatic actuatorbuilt in, the compact camera module can have additional features ofensured and stable focusing and zooming, and it performance can beenhanced.

For a zooming optical system, at least two groups of movable lens arerequired. In a compact zooming optical system of reduced dimension in adirection of the optical axis, an optical magnification is often variedin a stepwise manner where after the lens groups are separated from eachother, they are moved in position proximal to each other and thenseparated again. In accordance with the present invention, theelectrostatic actuator as described in relation with the fifthembodiment is used to independently move the two groups of lenses alongthe optical axis. Such an optical system can be implemented in anextremely compact design, and reliable and smooth operation can beensured.

Although some embodiments of the present invention have been describedwith reference to the accompanying drawings, it is not intended that theinvention should be limited to the precise form of them.

For example, the number of the groups of the activating electrodesincorporated in the electrostatic actuator is not limited to four like Ato D, or three lie A to C, unlike the description in the above, butthere may be five or more groups of the activating electrodes.

The polarity of the applied voltage to the electrodes also should not belimited to those which have been described in each embodiment, but it isimportant that a predetermined level of voltage may be applied betweenthe activating electrodes and the movable piece, and between the lowerelectrodes and the movable piece(s) so as to obtain a predeterminedlevel of electrostatic force required to attract a movable piece asdesired.

The timing according to which the voltage is applied to the electrodesshould also not be limited to those which have been described in eachembodiment, but voltage may simply be applied to the activatingelectrodes simultaneous with an application of voltage to the lowerelectrodes.

FIGS. 21 to 23 are timing charts representing sample timings that can beused in the embodiment of the present invention. These are all designedfor an application where four groups of the activating electrodes A to Dand the lower electrode E are incorporated.

The timing charts shown in FIG. 3 and FIG. 15 are based upon a sequencewhere adjacent ones of the groups of the activating electrodes (e.g., Aand B, or B and C) would be supplied with voltage when voltage is notapplied to the lower electrodes.

The present invention should not be limited to those described above,but alternatively, the sequence may be for an operation design whereonly one group of the activating electrodes would be supplied withvoltage when voltage is not applied to the lower electrode.

As to the sequence shown in FIG. 21, after the electrode A is suppliedwith voltage at time t1, the electrode A is turned to low in voltagelevel, at time t2, while the electrode B and the lower electrode E withvoltage. Further, subsequently, at time t3, while the electrodemaintains its voltage level unchanged, the lower electrode E is turnedto low in voltage level.]

After that, succeedingly applying voltage to the activating electrodes Ato D in order, the movable piece can be activated while keepingcontinuously attracted and almost fitted onto the activating electrodes.

Also, in such a case, as mentioned above in relation with the second tothird embodiments, applied voltage to the activating electrodes and thelower electrode(s) and a rate of areas among electrodes areappropriately adjusted to assuredly restrain vertical vibration of themovable piece.

In the sequence illustrated in FIG. 22, after the electrode B issupplied with voltage at time t1, the electrode B maintains its voltagelevel unchanged while the lower electrode E is supplied with voltage, attime t2. Subsequently, at time t3, the electrode B and the lowerelectrode E are turned to low in voltage level while voltage is appliedto the electrode C.

After that, succeedingly applying voltage to the electrodes in order,the movable piece can be activated while keeping continuously attractedand almost fitted onto the activating electrodes.

As to the sequence illustrated in FIG. 23, there is a “time delay”inserted in the timing for the activating electrodes A to D and thetiming for the lower electrode E. Voltage is applied to the activatingelectrodes A to D, respectively, in two successive timing units inorder, and when voltage is applied to the lower electrode in only asingle timing unit, the time delay is inserted in accord with eachrising time of every application of voltage to the remaining electrodes.Thus, the activating electrodes and the lower electrode(s) would neverbe supplied with voltage simultaneously, nor never be turned to lowsimultaneously.

In this way, even during a transition of applied voltage to the lowerelectrode E, the activating electrodes continuously keep supplied withconstant voltage, and therefore, “chattering” or “ripple” that may becaused by simultaneous application of voltage to the upper and lowerelectrodes or by interruption of the application of voltage can beeffectively suppressed.

As has been described in detail, in accordance with the embodiment ofthe present invention, supplying the lower electrode with voltagesimultaneous with application of voltage to the activating electrodes,the movable piece can laterally advance while continuously keepingattracted and almost fitted onto the activating electrodes.

Consequently, undesired vertical vibration of the movable piece can berestrained, and this attains almost the same effect of reduced clearancebetween the movable piece and the statical member, so that developmentof force exerted between those components can be facilitated. Since theclearance between the activating electrodes and the movable piece canalways keep minimized, strong and stable attracting force or Coulombforce can be developed. Moreover, a highly refined electrostaticactuator of reliable and stable operation can be attained, and this willbe lot of benefit to the industry.

While the present invention has been disclosed in terms of theembodiment in order to facilitate better understanding thereof, itshould be appreciated that the invention can be embodied in various wayswithout departing from the principle of the invention. Therefore, theinvention should be understood to include all possible embodiments andmodification to the shown embodiments which can be embodied withoutdeparting from the principle of the invention as set forth in theappended claims.

What is claimed is:
 1. An electrostatic actuator comprising: a firststatical member having an electrode array comprising at least threegroups of activating electrodes periodically deployed in a firstdirection; a second statical member facing the first statical member andhaving an electrode extending in the first direction; a movable pieceprovided between the first and second statical members; and a switchingcircuit configured to apply a first voltage to cause a potentialdifference between at least one of the at least three groups ofactivating electrodes and the movable piece and also configured to applya second voltage to cause a potential difference between the electrodeof the second statical member and the movable niece, the first voltagebeing applied to sequentially switch a destination of an applied voltagefrom at least one of the at least three groups of activating electrodesto another group in the first direction, and the second voltage beingintermittently applied while the first voltage is applied, wherein thepotential difference caused by applying the first voltage between theactivating electrodes and the movable piece is larger than the potentialdifference caused by applying the second voltage between the electrodeof the second statical member and the movable piece.
 2. An electrostaticactuator according to claim 1, wherein when the first voltage is appliedto at least one of the at least three groups of activating electrodesand the second voltage is applied to the electrode of the secondstatical member, a face-to-face area of at least one of the at leastthree groups of activating electrodes and the movable piece is largerthan a face-to-face area of the electrode of the second statical memberand the movable piece.
 3. An electrostatic actuator according to claim1, wherein the first voltage is applied to cause a potential differencebetween at least two adjacent groups of the at least three groups ofactivating electrodes and the movable piece.
 4. An electrostaticactuator according to claim 1, further comprising a stopper positionedbetween the first statical member and the movable piece.
 5. Anelectrostatic actuator comprising: a first statical member having anelectrode array comprising at least three groups of activatingelectrodes periodically deployed in a first direction; a second staticalmember facing the first statical member and having an electrodeextending in the first direction; a movable piece provided between thefirst and second statical members; and a switching circuit configured toapply a first voltage to cause a potential difference between at leastone of the at least three groups of activating electrodes and themovable piece and also configured to apply a second voltage to cause apotential difference between the electrode of the second statical memberand the movable piece, the first voltage being applied to sequentiallyswitch a destination of an applied voltage from at least one of the atleast three groups of activating electrodes to another group in thefirst direction, and the second voltage being intermittently appliedwhile the first voltage is applied, wherein when applying the secondvoltage, the switching circuit applies a voltage of a polarity reversedto that of the first voltage to at least one of the at least threegroups of activating electrodes to which the first voltage is notapplied.
 6. An electrostatic actuator comprising: a first staticalmember having an electrode array comprising at least three groups ofactivating electrodes periodically deployed in a first direction; asecond statical member facing the first statical member and having anelectrode extending in the first direction; a movable piece providedbetween the first and second statical members; and a switching circuitconfigured to apply a first voltage to cause a potential differencebetween at least one of the at least three groups of activatingelectrodes and the movable piece and also configured to apply a secondvoltage to cause a potential difference between the electrode of thesecond statical member and the movable piece, the first voltage beingapplied to sequentially switch a destination of an applied voltage fromat least one of the at least three groups of activating electrodes toanother group in the first direction, and the second voltage beingintermittently applied while the first voltage is applied, wherein theswitching circuit applies a voltage so that a first condition and asecond condition are alternatively repeated, the first condition beingthat a potential of each group of activating electrodes being higherthan that of the movable piece, and the second condition being that apotential of each group of activating electrodes being lower than thatof the movable piece.
 7. An electrostatic actuator comprising: a firststatical member having an electrode array comprising at least threegroups of activating electrodes periodically deployed in a firstdirection; a second statical member facing the first statical member andhaving an electrode extending in the first direction; a movable pieceprovided between the first and second statical members; and a switchingcircuit configured to apply a first voltage to cause a potentialdifference between at least one of the at least three groups ofactivating electrodes and the movable piece and also configured to applya second voltage to cause a potential difference between the electrodeof the second statical member and the movable piece, the first voltagebeing applied to sequentially switch a destination of an applied voltagefrom at least one of the at least three groups of activating electrodesto another group in the first direction, and the second voltage beingintermittently applied while the first voltage is applied, wherein apulse time width during which the switching circuit applies the firstvoltage is longer than a pulse time width during which the switchingcircuit applies the second voltage.
 8. An electrostatic actuatorcomprising: a first statical member having an electrode array comprisingat least three groups of activating electrodes periodically deployed ina first direction; a second statical member facing the first staticalmember and having an electrode extending in the first direction; amovable piece provided between the first and second statical members;and a switching circuit configured to apply a first voltage to cause apotential difference between at least one of the at least three groupsof activating electrodes and the movable piece and also configured toapply a second voltage to cause a potential difference between theelectrode of the second statical member and the movable piece, the firstvoltage being applied to sequentially switch a destination of an appliedvoltage from at least one of the at least three groups of activatingelectrodes to another group in the first direction, and the secondvoltage being intermittently applied while the first voltage is applied,wherein the switching circuit starts applying the first voltage prior toapplying the second voltage, and the switching circuit stops to applythe first voltage after stopping to apply the second voltage.
 9. Anelectrostatic actuator comprising: a first statical member having anelectrode array comprising at least three groups of activatingelectrodes periodically deployed in a first direction; a second staticalmember facing the first statical member and having a first electrodeextending in the first direction and a second electrode extending in thefirst direction almost parallel with the first electrode; a firstmovable piece provided between the first and second statical members; asecond movable piece provided between the first and second staticalmembers; and a switching circuit configured to apply: (i) a firstvoltage to cause a potential difference between at least one of the atleast three groups of activating electrodes and the first movable piece,(ii) a second voltage to cause a potential difference between the firstelectrode and the first movable piece, the first voltage being appliedto sequentially switch a destination of an applied voltage from at leastone of the at least three groups of activating electrodes to anothergroup in the first direction, the second voltage being intermittentlyapplied while the first voltage is applied, (iii) a third voltage tocause a potential difference between at least one of the at least threegroups of activating electrodes and the second movable piece, and (iv) afourth voltage to cause a potential difference between the secondelectrode and the second movable piece, the third voltage being appliedto sequentially switch a destination of another applied voltage from atleast one of the at least three groups of activating electrodes toanother group in the first direction, and the fourth voltage beingintermittently applied while the third voltage is applied.
 10. Anelectrostatic actuator according to claim 9, wherein the potentialdifference caused by applying the first voltage between at least one ofthe at least three groups of activating electrodes and the first movablepiece by is larger than the potential difference caused by applying thesecond voltage between the first electrode and the first movable piece,and the potential difference caused by applying the third voltagebetween at least one of the at least three groups of activatingelectrodes and the second movable piece is larger than the potentialdifference caused by applying the fourth voltage between the secondelectrode and the second movable piece.
 11. An electrostatic actuatoraccording to claim 9, wherein when the first voltage is applied to atleast one of the at least three groups of activating electrodes and thesecond voltage is applied to the first electrode, a face-to-face area ofat least one of the at least three groups of activating electrodes andthe first movable piece supplied with the electrode array is larger thana face-to-face area of the first electrode and the first movable piece,and when the third voltage is applied to at least one of the at leastthree groups of activating electrodes and the second voltage is appliedto the second electrode, a face-to-face area of the at least one of theat least three groups of activating electrodes and the second movablepiece is larger than a face-to-face area of the second electrode and thesecond movable piece.
 12. An electrostatic actuator according to claim9, further comprising a power supply circuit configured to apply a fifthvoltage to the first electrode to cause a potential difference largerthan that which is caused between the first electrode and the firstmovable piece by applying the second voltage, whereby while the firstmovable piece can be held in a fixed position, the second movable piececan be advanced.
 13. An electrostatic actuator according to claim 9,wherein the first and the third voltages are applied to cause apotential difference between at least two adjacent groups of the atleast three groups of activating electrodes and the first and secondmovable pieces, respectively.
 14. An electrostatic actuator according toclaim 9, wherein a pulse time width during which the switching circuitapplies the first and the third voltages is longer than a pulse timewidth during which the switching circuit applies the second and thefourth voltages to the first and the second electrodes.
 15. Anelectrostatic actuator according to claim 9, wherein the switchingcircuit starts applying the first and the third voltages prior toapplying the second and the fourth voltages, and the switching circuitstops to apply the first and the third voltages after stopping to applythe second and the fourth voltages.
 16. An electrostatic actuatoraccording to claim 9, further comprising a stopper positioned betweenthe first statical member and the first and second movable pieces. 17.An electrostatic actuator according to claim 9, wherein the firstvoltage and the third voltage are in common, and the second voltage andthe fourth voltage are in common.